The invention relates to measuring cell voltages of a fuel cell stack.
A fuel cell is an electrochemical device that converts chemical energy produced by a reaction directly into electrical energy. For example, one type of fuel cell includes a proton exchange membrane (PEM), often called a polymer electrolyte membrane, that permits only protons to pass between an anode and a cathode of the fuel cell. At the anode, diatomic hydrogen (a fuel) is reacted to produce hydrogen protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the hydrogen protons to form water. The anodic and cathodic reactions are described by the following equations: EQU H.sub.2.fwdarw.2H.sup.+ +2e.sup.- at the anode of the cell, and EQU O.sub.2 +4H.sup.+ +4e.sup.-.fwdarw.2H.sub.2 O at the cathode of the cell.
Because a single fuel cell typically produces a relatively small voltage (around 1 volt, for example), several fuel cells may be formed out of an arrangement called a fuel cell stack to produce a higher voltage. The fuel cell stack may include plates (graphite composite or metal plates, as examples) that are stacked one on top of the other, and each plate may be associated with more than one fuel cell of the stack. The plates may include various channels and orifices to, as examples, route the reactants and products through the fuel cell stack. Several PEMs (each one being associated with a particular fuel cell) may be dispersed throughout the stack between the anodes and cathodes of the different fuel cells.
The health of a fuel cell stack may be determined by monitoring the individual differential terminal voltages (herein called cell voltages) of the fuel cells. In this manner, a particular cell voltage may vary under load conditions and cell health over a range from -1volt to +1 volt. The fuel cell stack typically may include a large number of fuel cells, and thus, common mode voltages (voltages with respect to a common voltage (ground)) of the terminals of the fuel cells may be quite large (i.e., some of the voltages of the terminals may be near 100 volts, for example). Unfortunately, semiconductor devices that may be used to measure the cell voltages typically are incapable of receiving high common mode voltages (voltages over approximately 18 volts, for example). One solution may be to use resistor dividers to scale down the terminal voltages. However, the tolerances of the resistors may introduce measurement errors.